The present invention is directed towards an improved pitch changer and more particularly to a pitch changer which minimizes the occurrance of "clicks" or "glitches" in the audio output signal derived by the pitch changer. Devices of this general type are generally known as "multiplicative pitch (or frequency) shifter devices", characterized by the fact that any input signal is multiplied in frequency by an amount determined by the user, typically over a continuously variable range between 0.25 and 4.0 corresponding to shifts of -2 to +2 octaves. Such devices are capable of accepting a baseband audio signal, typically from 20 Hz to 20 kHz, regardless of the complexity of the signal (i.e., the number of simultaneously present frequency components), and performing the pitch changing operation.
Devices of this type normally use digital techniques to achieve the desired pitch change. The audio input signal is read into a random access memory at a fixed rate and read out of the memory at an increased or decreased rate so as to increase or decrease the pitch, respectively. Because the variable rate readout takes the samples from the memory at a different rate than the samples are read into the memory, both the pitch (frequency) and time duration (phase) of the audio input signal will be varied. Since the time of duration of the audio output signal generated by the pitch change is different from that of the input signal, some means must be provided to effectively increase the duration of the output signal when the pitch changer is increasing the frequency of the input signal and for decreasing the time duration of the output signal when a pitch changer is decreasing the frequency of the input signal.
Normally this is done by either deleting or repeating segments of the digitized audio input signal stored in the pitch changer memory. For example, if the pitch ratio is increased, the memory of the frequency changer will "run out" of samples to be transmitted before the desired time duration of the tone is completed. For this reason, the pitch changer normally returns to the beginning of the signal segment stored in the pitch changer memory and appends the samples corresponding to this segment to the output data stream (at the new, higher sampling rate), until such time as new input samples become available. For a decrease in pitch ratio, samples are discarded instead of repeated. In either case, one portion of the sample segment stored in the pitch changer memory is effectively spliced to another non-contiguous portion thereof.
The primary problem with pitch changers of the foregoing type is that the splice between signal segments is normally audible and usually objectionable. This splice can sound like periodic "clicks" of varying amplitude or phase, or various obvious, but readily discernable, "glitches" in the output signal. To remedy this problem, the prior art has suggested three basic schemes: (1) timing the splice so that it occurs on signal zero crossings or in zero crossings in the same direction; (2) smoothing the splice by slowly overlaying two segments; or (3) filtering out the spurious frequency components generated by the splice. All of these methods are less than optimal since there is little or no attempt to match the splice to the actual signal characteristics and select the best possible splicing point. The primary object of the present invention is to select a splicing point which will produce the least number of "clicks" or "glitches" in the audio output and to use this splicing point whenever stored sample signals must be deleted or repeated.